1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic printing method.
Here, examples of an electrophotographic image forming apparatus include a copying machine, a laser beam printer, an LED printer, and a facsimile apparatus.
2. Description of the Related Art
FIG. 2 is a schematic diagram of a conventional image forming apparatus employing an electrophotographic printing method.
In the image forming apparatus employing the electrophotographic printing method such as a copying machine and a laser beam printer, a photosensitive drum 201, which is an electrostatic latent image bearing member, is uniformly charged and is irradiated with a laser beam 203 which corresponds to image information, to thereby form an electrostatic latent image. In the image forming apparatus, the electrostatic latent image is exposed by supplying the electrostatic latent image with a developer (hereinafter, referred to as “toner”) as a recording material, by using developing means 204, and then the image is transferred onto a recording medium 210 from the photosensitive drum, to thereby form an image on the recording medium.
The image forming apparatus includes a charging device 202 for uniformly charging the photosensitive drum 201 and an exposure device 203 for forming the electrostatic latent image corresponding to printing information and image information through laser exposure onto the charged photosensitive drum 201, used for reproducing the image information on the photosensitive drum 201. The image forming apparatus further includes a developing roller 204 for visualizing the formed electrostatic latent image with a developer (i.e., toner).
In addition, the image forming apparatus includes a transfer device 205 for transferring a toner image reproduced on the photosensitive drum 201 onto the recording medium 210, a cleaning device 206 for removing transfer residual toner on the photosensitive drum 201, and a fixing device 207 for permanently fixing the toner image transferred on the recording medium.
Image forming processes of the conventional image forming apparatus are carried out at a timing shown in FIG. 3.
In FIG. 3, the same areas on the photosensitive drums are represented by vertical lines. FIG. 3 shows a timing of a print instruction, a timing of a rotation of the photosensitive drum, and a relationship among charging bias (i.e., voltage applied to a charging member) of a corresponding area, a surface potential of the photosensitive drum, and a transfer bias (i.e., voltage applied to the transfer member) which are measured immediately after the charging.
Each interval between scale lines indicated by the broken lines parallel to an ordinate axis represents half the circumference of the photosensitive drum. An abscissa axis represents a length of elapsed time.
First, an image forming apparatus main body receives a print instruction from an external computer or the like (301), and then the photosensitive drum starts rotating (302).
After that, a charging bias of −1000 V is applied to the charging device (303) to uniformly charge a surface of the photosensitive drum to a charged potential VD of −500 V (304).
In this case, a predetermined constant charging bias is applied to a charging roller regardless of whether it is before an image forming process, during the image informing process, or during an interval between image forming processes.
After that, an electrostatic latent image is formed on the photosensitive drum having been charged to the charged potential VD, and the surface potential of an exposed portion on the photosensitive drum becomes an exposure potential VL (FIG. 3 shows a case where image exposure is not carried out, for convenience).
When the electrostatic latent image formed on the photosensitive drum reaches the developing roller, the electrostatic latent image is subjected to development to be visualized on the photosensitive drum as a toner image.
When the toner image visualized on the photosensitive drum reaches the transfer roller, the toner image is applied with a predetermined transfer bias (305), thereby being electrostatically transferred onto a recording medium supplied from a cassette serving as a feeding apparatus in synchronization with the print instruction.
Then, the recording medium having the toner image transferred thereon is transported to the fixing device, and the toner image is applied with heat and pressure, thereby being fixed on the recording medium as a permanent image.
When transfer residual toner on the photosensitive drum having passed through the transfer roller reaches the cleaning device, the transfer residual toner is removed by the cleaning device from the surface of the photosensitive drum. Then, the surface of the photosensitive drum is charged again by the charging roller to be readied for the subsequent image formation.
Examples of the above-mentioned image forming apparatus include one in which, as a control of the transfer bias, the transfer bias is applied to a non-image forming area, which is generated during an interval between the recording media (306) or at a time of an initial rotation before printing (307), to perform correction of the transfer bias (see Japanese Patent Application Laid-Open No. H10-207262). In the image forming apparatus, the transfer bias is applied to the non-image forming area, and a transfer current or a transfer voltage obtained at that time is monitored, to thereby detect a change in resistance of the transfer roller serving as a transfer member, based on the current value and the voltage value. According to the detected change in resistance of the transfer roller, the transfer bias at the time of transfer of the toner image is corrected.
However, in the image forming apparatus in which the transfer member, of which a surface is composed of a foam, is directly abutted against the image bearing member, and in which voltage is applied to the transfer member even when the recording material is not present in a transfer portion, there arises a problem of excessive charging in which a part of the potential of the charged image bearing member becomes a predetermined potential or more.
Even when the recording material is not present in the transfer portion as shown in the above-mentioned conventional example, in a case where voltage is continuously applied to the transfer member, the potential of the image bearing member having passed through the transfer portion is generally lowered by elimination of charge under the influence of voltage. In the transfer portion, the problem of the excessive charging is not raised when the potential of the image bearing member is uniformly reduced by elimination of charge.
However, when the surface of the transfer member is composed of a foam, a difference in amount of a current flowing from the transfer member to the image bearing member is generated between a void portion (i.e., cell portion) and a non-void portion (i.e., non-cell portion) of the transfer roller.
When the image bearing member passes through the transfer portion, in an area on the image bearing member which corresponds to the non-cell portion of the transfer member, a current can easily flow from the transfer member to the image bearing member, and the potential of the image bearing member is affected by the transfer bias, thereby reducing the potential of the image bearing member by elimination of charge. On the other hand, when the image bearing member passes through the transfer portion, in an area on the image bearing member which corresponds to the cell portion of the transfer member, a current cannot easily flow as compared with the case where the area corresponds to the non-cell portion. As a result, the potential of the image bearing member is hardly affected by the transfer bias, so the potential is not changed in the transfer portion.
The potential of the area on the image bearing member which corresponds to the cell portion is not changed in the transfer portion. For this reason, the area reaches the charging portion again while maintaining the potential obtained through the previous charging by the charging member, thereby being charged to the same potential by the charging member. Thus, in a case where almost no potential difference is generated between potentials before and after the charging by the charging member, a phenomenon called excessive charging is caused in which the charge on the area is increased to the desired potential or more. The area in which the excessive charging has been caused has a desired potential or more, so there may arise a problem in that white dots appear at the time of image formation (particularly at the time of formation of a halftone image).